Blocked from making calls and calling call centre

I have been trying to make calls and have enough airtime and all the time I will receive a message I have insufficient airtime and my important call will be cancelled. I end up not talking to anyone at this time of the year. I have been trying to call their call centre number the call just gets ended without even ringing. I went to a shop to place a complaint a lady at the call centre named Charlotte said I may have been blocked. How is a customer blocked from contacting a call centre, from voicing a problem, why was i blocked because I hardly call the number, yesterday was the first time. I need to make calls. I complained because MTN was just fleecing me airtime in the name of promotions instead i was losing airtime i bought. I can't even buy airtime now via cellphone banking because i am said to have no airtime yet my airtime is R26